Transient behaviour simulation of large, explosive, and ignimbrite forming eruptions by a multiphase thermo fluid dynamic model.

A multiphase thermo fluid dynamic model has been improved to assess the effect of a range of particle size on the transient two dimensional behaviour of large, explosive, and ignimbrite forming eruptions. The model accounts for mechanical and thermal non-equilibrium conditions between a continuous gas phase and N solid phases characterized by specific physical properties. The dynamics of the process was simulated by adopting a grid scale approach able to resolve the meso scale features of the flow and the high sub-grid gas turbulence. Viscous and interphase effects were expressed in terms of newtonian stress tensors and gas-particle or particle particle coefficients. Several numerical simulations of such collapsing explosive eruptions were carried out. Their dynamics depict the formation of the vertical jet, the column collapse, the building of the pyroclastic fountain, the generation of radially spreading pyroclastic current, and the development of thermal convective instabilities in the fountain and in the current. The results highlight the importance of the multiphase development of the model and describe several mechanical and thermal non-equilibrium effects. Low concentration zones tend to follow the dynamics of the hot ascending gas, both in the convective plume above the fountain, and in the propagation of the associated pyroclastic current. High concentration parts tend to sediment mainly along the ground, both in the proximal area for the mixing of material in the fountain, and in the boundary layer for the loss of momentum.